Federal prosecutors have charged two former JPMorgan Chase traders with securities fraud. The two men worked in London. And they are part of the so-called London Whale case, which cost the company more than $6 billion. U.S. officials say the men lied about the value of some derivatives trades to cover up mounting lawsuits. More from NPR's Jim Zarroli.

JIM ZARROLI, BYLINE: The two men were identified as Julien Grout and his supervisor Javier Martin-Artajo. They worked in JPMorgan's chief investment office, which was responsible for investing some $350 billion in excess deposits, money that wasn't being lend out to the company's customers. They invested part of that money in the complex and highly lucrative world of derivatives. U.S. Attorney Preet Bharara spoke at a news conference today.

PREET BHARARA: While the transactions and financial products involved may be complex, the criminal conduct alleged is simple and straightforward.

ZARROLI: U.S. officials said that starting in early 2012, a portfolio overseen by the men charged today began to rack up losses. And higher-ups in the office expressed concern. So Martin-Artajo allegedly began to order traders to value some of the assets in the portfolio in a way that would make the losses look smaller. Over the next few weeks, Bharara says, traders essentially kept two sets of books: one for themselves and one for outsiders.

BHARARA: The hope was that the bets would turn around, but they didn't. And as the bets continued to sour, rather than scale back and come clean, the defendants allegedly doubled down not only on their bets, but also on their fraudulent valuations and on their cover-up.

ZARROLI: U.S. officials say the men were able to get away with this in part because the company wasn't trying all that hard to prevent it. JPMorgan Chase had an office that was supposed to be checking their numbers, but officials say it consisted of just one London-based employee. FBI special agent April Brooks.

APRIL BROOKS: You would think one of the biggest banks on the street would have rigorous compliance and oversight over their books. Think again.

ZARROLI: It wasn't until early April that the company began to scrutinize the books more carefully, and that was only because press reports came out about the huge trades taking place. The articles indentify JPMorgan Chase trader Bruno Iksil as the man responsible for the trading, calling him the London whale.

As it turns out, U.S. officials say Iksil had tried to sound an alarm about what was happening. They say he repeatedly tried to convince Martin-Artajo, his supervisor, to come clean about the losses. But they say he was rebuffed. Iksil is now a cooperating witness for the government.

As for the men charged today, they haven't yet been arrested. Grout returned to his native France after leaving JPMorgan. Martin-Artajo, who lives in London, is said to be on vacation. His lawyers said they were confident he will be cleared when a fair reconstruction of events is completed. Jim Zarroli, NPR News, New York. Transcript provided by NPR, Copyright NPR.